Poisoning and reactivation processes in oxide-type cathodes: Part I. Polycrystalline mixed oxides

Applications of Surface Science 8 (1981) 125—144 North-holland Publishing Company

POISONING AND REACTIVATION PROCESSES IN OXIDE-TYPE CATHODES: PART I. POLYCRYSTALLINE MIXED OXIDES A. SHIH and G.A. HAAS Naval Research Laboratory, Washington, DC 20375, USA Received 3 September 1980

A study has been made of the poisoning and reactivation characteristics of alkaline earth oxide-type cathodes after extended periods of shelf storage. Both emitted and incident electrons were used to measure changes in the electronics properties, i.e. work function. The variations in work function over the surface were obtained in both distribution form as well as topographic presentation using a scanning low energy electron probe (SLEEP). These measurements were correlated with simultaneously occurring compositional changes using Auger, gas desorption and ion scattering techniques. Measurements were made on realistic cathodes in actual vacuum tube ambients. The results showed that oxide-type cathodes poison within a few hours after shut-down by the adsorption of residual gases contained in the vacuum ambient. (The effects of CO 2 were specifically demonstrated.) These adsorbates are, however, desorbed upon heating and in combination with other reactivation processes (such as formation of surface Ba layers when using reducing substrates), the cathode can reach full activation again by the time the temperature reaches the normal operating temperature. The poisoning and reactivation phenomena are a combination of a number of simultaneous processes, and studies to separate and identify these is the objective of past II of this paper.

1. Introduction As has been pointed out recently [1,2], the development of expendable tubes for various military applications in the last few years has placed stringent new requirements on cathode performance. Among the most demanding of these is that the tube may sit dormant for periods of months (or even years) and then, following a turn-on command, must reach full operational levels within a few seconds. Studies [1,2] which have just been completed of various impregnated and oxide-type cathodes used under these conditions have shown that all cathodes tested (B, M, tungstate, scandate and coated oxide powder) poison during tube shut-down. An example of this poisoning effect as observed with oxide cathodes is shown in fig. I where the work function distribution is plotted for the active state (immediately after tube shut-down) as well as after an ambient exposure of a few hours. (These data were taken at 400 K with incident electrons using the scanning low energy electron probe (SLEEP) method [3].) The rise in average work function observed during 0 378-5963/81/0000—-0000/$ 02.75 © 1981 North-Holland

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A. Shili, GA. Haas / Poisoning and reactivation in oxide-type cathodes. Part I.

ACTIVE

AFTER AMnENT

WORK FUNCTION (eVI

Fig. 1. Work function distributions obtained from oxide-type cathode in the active state and after poisoning by ambient exposure. The distributions are derived from work function values of 30000 samples obtained during the 3 s required for one scan over the surface using the SLEEP system.

ambient exposure, as shown in fig. 1, is reversible in that reheating the cathode to operating temperatures of ~l 250 K will usually decrease the work function back to its active state. Measurements of the change in work function during the reheating of these (as well as other cathodes after they had been subjected to similar vacuum ambient exposures) clearly demonstrated [21 that the achievement of rapid turn-on properties not only involved raising the cathode temperature in the prescribed time interval but, more importantly, it also required the successful reactivation of the cathode from the poisoned state as well. Since this latter effect was shown to be the doniinant one in limiting the rate of cathode activity during turn-on, a more intensive investigation of the poisoning and reactivation processes was begun. Because oxide cathodes appeared to show the greatest promise for use in present-day expendable tube applications [1,21, the poisoning and reactivation mechanisms of these cathodes were the first to be studied as part of this investigation, and it is this work that wifi be described here. Oxide cathodes have been extensively studied in the past [4—61, as have some “oxide-type” cathodes where the Ni, apart from being the substrate, is also dispersed in the oxide powder [7—11].It is only very recently, however, that modern surface analysis techniques have been applied to studies of these cathodes [12—16]. In part I of this paper, these techniques are used in studying changes in the work function of oxide-type cathodes as obtained from thermionic emission and SLEEP measurements. These work function changes are then correlated with surface chemical changes determined with Auger electron spectroscopy (AES) and ion scattering spectroscopy (ISS) techniques. In addition to these, correlations were also made with gas desorption measurements as well as the use of multiple Auger energies to

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determine gradients in surface chemical composition. This latter new technique, in conjunction with ISS, will be shown to be particularly useful, for example, in the study of Ba surface monolayers when active Ni substrates are used. The major emphasis in part I of this study, therefore, is to use these modern techniques to characterize some of the dominant poisoning and reactivation processes in realistic oxidetype cathodes under actual vacuum tube shelf-life and rapid turn-on conditions. Part II of this paper is a continued investigation of this subject with emphasis on the understanding of the basic mechanisms underlining some of the phenomena observed. In order to separate the multiplicity of poisoning and reactivation processes that were found to occur in part i, in part II only specific components of the ambient gases were investigated, one at a time, on well-defined single crystal oxide films. Use was also made of the interference of the incident electron wave with the lattice periodicity to provide information of changes in surface band structure during the different poisoning and reactivation processes. The results, which are displayed in terms of low energy electron reflection (LEER) patterns permit the work function changes (such as observed in part I) to be described in terms of their components, namely changes in Fermi level and surface dipole contributions [13]. These changes are then correlated with surface chemical composition data using multiple Auger energies, as was the case in part I. From characteristic changes in the LEER patterns, changes in the lattice structure and chemical composition are also identified and correlated with gas desorption studies. Descriptions of experimental techniques as well as methods of data work-up will be kept to a minimum in both parts of this paper since these have been extensively discussed in the previous publications of the authors cited in the references. The results of the data obtained in part I, as well as part II of this study, while obviously not including all possible effects, provide an understanding of many of the important basic mechanisms involved in the shelf-life poisoning and reactivation processes occurring in oxide-type cathodes. With this understanding, it becomes considerably clearer which direction one must follow to minimize the effects of poisoning as well to provide improved reactivation methods for present and future expendable tube applications requiring high emission and very rapid turn-on characteristics. A study, similar to this oxide cathode study, is currently in progress involving impregnated-type cathodes since the use of these types of cathodes may be required in forthcoming expendable tube applications where current densities in excess of 2 are needed. 5 A/cm 2. Poisoning during ambient exposure; example of C pick-up In fig. I it was demonstrated how the work function of an oxide-type cathode increased after a few hours of tube shut down (p 10—8 Torr range). While it is realized that multiple poisoning processes are undoubtedly contributing to the in-

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crease in work function of the oxide cathodes, the pick-up of C was found to have a dominant correlation with the increase in work function, much as was also noted earlier for the case of impregnated cathodes [1]. In order to see, however, if the appearance of the C did indeed play a contributing role in the increase in work function, the following combined SLEEP and scanning Auger microprobe (SAM) experiments were performed. An oxide cathode made from a sprayed triple carbonate mixture was converted and activated in the standard manner until an “active” work function distribution of the type shown in fig. 1 was obtained. Then after allowing the cathode to sit in the tube ambient for several hours and observing the rise in work function, typified as well in fig. 1, the usual increase in surface C Auger peaks was also seen. (It should be noted that the C peak was only observable with a widely rastered beam having a beam current of a fraction of a microamp.) At this point, a portion of the surface

p--I~~ Fig. 2. Scanning Auger micro~aphs(SAM) and SLEEP work function topograph of oxide cathode which had been poisoned by a C overlayer during ambient exposure. The upper left and right figures are SAM results of C and 0, respectively, while the bottom figure is the work function display of the same region of the surface obtained with SLEEP (white is low ~). The surface C layer has been removed from the upper left rectangular portion by an electron beam showing BaSrO and low ~ beneath. (The horizontal dimension is —0.3 mm.)

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was scanned with a standard Auger beam for which it was observed the C Auger peak quickly disappeared due to desorption of the surface C. Rerastering a larger area with the reduced beam current provides the upper two SAM photographs of fig. 2, the left one being for C while the right one is for 0 (although it is also identical to the Ba or Sr SAM photo). The rectangular portion of the surface area which had been subject to electron desorption is shown in the upper left region of the photographs. (The perpendicular arrows point from the lower right corner of this region.) The removal of the surface C is clearly evident from the dark appearance of the rastered region in the upper left C SAM photograph of fig. 2. The upper right photo of fig. 2, on the other hand, shows the presence of Ba and Sr oxides beneath the surface C, which are now exposed. The lower photograph is a SLEEP work function topograph of the same region of the oxide cathode surface as shown in the SAM photos. The light regions of the SLEEP topograph are low work function regions (~2eV for this case), with the range from light to dark being ~l eV. The fact

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that the work function is high in the areas where the surface C remains, but is reduced essentially to the value of the active states where the C is desorbed, points to the direct correspondence that the surface C layer has with increasing the work function. When the surface described above is heated, very little change occurs up to ~750 K. This is shown in the left-hand SLEEP data of fig. 3. Here, the lower graphs give the work function in quantitative distribution form from 30000 samples of the region shown in the topographical picture above them. It is noted that the work function distribution of the “poisoned” surface is very similar to the one shown in fig. 1 except for the additional low work function contribution in fig. 3 where the C was removed by electron desorption. Heating beyond ~750 K, however, causes the adsorbed C to disappear from the SAM picture and the work function from the SLEEP data to become more uniform. Upon heating to 1325 K, all C is gone from the SAM pictures and the absolute value of the work function as well as the distribution of it has returned to that of the active state (cf. lower right curve of fig. 3 with fig. 1); furthermore, the high work function (non-rastered) region now has the same work function as the rest of the surface, as seen in the upper right photograph of fig. 3. (It will be shown in the next section that by 1325 K, essentially all alkaline earth carbonates which may have formed on the surface during ambient exposure will have decomposed back to the oxide.)

3. Reactivation effects after shelf-life; General work function changes and gas desorption Typical examples of changes in work function of oxide-type cathodes when heated after shelf-life exposure are shown in fig. 4. Curve (a) represents results obtained from a standard oxide cathode where the work function values were obtained using the incident electron method of the SLEEP technique. The data were taken at ~~~400 K, after the cathode had been heated for 2 mm at the progressively higher temperatures noted in the graph. (While the “peak” in work function at ~700 K was usually seen, it was not always as pronounced as shown in the example of fig. 4a.) Curve (b) was obtained following a similar ambient exposure of a coated powder cathode (CPC), as is described in ref. [1]. Here, however, the work function was obtained from extrapolated pulsed thermionic emission data at the actual temperatures specified. Each data point represents heating at that temperature for 5 s. Since the results of curve (b) were obtained with thermally emitted electrons, the temperature range of measurements is considerably narrower than curve (a). It is difficult to interpret all the features of fig. 4 because, as will be shown later, there are so many simultaneous processes taking place (especially at the lower temperature). However, it can be said that the majority of the activation effects seem to occur between ~a700K to 900 K (curve (a)) and that beyond ~l 100 K the work function has essentially reached the reactivated state (i.e., is within ~80 mV of it, as seen on the expanded scale of curve (b)).

A. Shih, G.A. Haas / Poisoning and reactivation in oxide-type cathodes. Part I. 4.0

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The nearly complete reactivation of these cathodes which occurs at temperatures well below the normal operating temperature of ~1250 K, is the characteristic that makes the oxide-type cathodes so desirable for use in the fast turn-on application of expendable tubes [2] with low and moderate cathode loading. One of the interesting results noted from the gas analysis studies during reactivation [2] was that the chemical affinity (i.e. sticking coefficient) for ambient CO2 gas during shelf-life was much greater for oxide-type cathodes than any of the other cathodes studied for expendable tube use (see fig. 8 of ref. [2]). If it is postulated that the affinity for residual CO2 is caused by the formation of a surface carbonate, then the removal of the C layer and accompanying reactivation, as seen in figs. 3 and 4, should be related to the decomposition of the alkaline earth carbonates. The CO2 desorbed from oxide-type cathodes during reactivation after shelf-life was therefore compared to previous data obtained from other experiments involving carbonate conversions. The results of this are plotted in fig. S where the equilibrium

A. Shih, G.A. Haas / Poisoning and reactivation in oxide-type cathodes. Part I.

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dissociation temperature of Ba and Sr carbonates are plotted for over 15 orders of magnitude variation in ambient CO2 pressure. (Similar hydroxide dissociation temperatures are plotted for variations in H2O pressure.)

A. Shih, G,A. Haas / Poisoning and reactivation in oxide-type cathodes. Part I.

133

The circles and squares in fig. 5 show the typical decrease in dissociation temperature with decreasing equilibrium vapor pressures for carbonate powder samples [4,17]. The solid curve just below these is a plot of the CO2 pressure taken during 4 Torr conversion ambient COto the oxide of sprayed Ba, Sr carbonate cathodes at l0~—i0 2 pressures [18]. It is seen that the peaks due to the Sr and Ba carbonate dissociation (at these slightly lower pressures) fall well along the extrapolation of the above powder samples. Near the bottom of the graph is plotted the dissociation temperature (arrow) for BaCO3 films in the UHV region of 10_12_1013 Torr partial pressure of CO2 used in the LEER experiments to be described in part II. (The dashed curve merely joins the low vacuum to high vacuum data.) Typical results in the l0~ to 10—10 Torr region of the present experiments were obtained from measurements of desorbed CO2 (i.e. mass 44) during reheating after ~2½ h shelf-life exposure. These data are shown as the solid curve in this pressure region. Similar results are also plotted for the hydroxide conversion to the oxide [2, 1 8, 19]. It is noted that the data for the CO2 desorption during reactivation following the shelf storage, fall close to the dashed curves of the Ba and Sr carbonate dissociation. The temperature of this dissociation (‘-‘700—900 K) is also in agreement with the SAM and SLEEP data previously discussed.

4. Reactivation effects following sputtering; Formation of Ba surface monolayer when using active Ni substrates From previous results [15] it was shown that (based on Auger measurements) the amount of 0 present at the oxide—Ni interface of cathodes with passive Ni was just that required to form the alkaline earth oxide. Cathodes with active Ni, on the other hand, showed as excess of 0 at the interface resulting from the oxidation of the reducing additives in the Ni. For example, in the case of W additives the amount of this excess 0 detected at the interface was in agreement with the reduction of BaO to form the tungstates and to produce free Ba from the following two reactions (cf. fig. 10 and eqs. (6) and (7) of ref. [15]): 6BaO+W-~Ba3WO6‘F 3Ba,

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The presence at the surface of any of the free Ba generated from these reactions, however, could never be verified from standard AES sputter profile measurements because such measurements only showed variations in Ba concentretion which were within the realm of experimental error. Because of the known limits of AES surface sensitivity and since effects of the reducing additives in the substrates were observed from the decrease in the work function [15], a series of experiments usingincreased surface sensitivity were undertaken to soc if any of the excess Ba generated by reactions 1 and 2 could be detected.

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In order to eliminate extraneous effects, such as reactivation from poisoning by ambient gases, the experiments described in this section involve only the thermal activation of a sputter-cleaned alkaline earth oxide surface. It was observed that the work function of a sputter-cleaned surface would start to decrease to a more “active state” (lower ~ value) when heated between ~—700— 800 K. Typical results of this activation effect are given in the upper curve of fig. 6. The fact that this “active state” could be created by heating, removed by surface sputtering, reactivated by heating, etc., indicated that the “activation” was associated with a surface-type layer. (This “activation” is coincidentally in the same temperature range as the reactivation from ambient CO2 pick-up, discussed in the previous section, and illustrates mentioned earlier.)

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Standard AES data from monitoring the Ba 590 V (MNN) transition showed little change until a temperature of about 1000 K was reached (lower curve, fig. 6). On the other hand, compositional data obtained using the more surface sensitive AFTER SPUTTERING

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A. Shih, G.A. Haas / Poisoning and reactivation in oxide-type cathodes. Part I.

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ISS techniques started showing surface Ba concentration changes commensurate with the work function changes (i.e. near 700—800 K). It is interesting to note that AES data, taken with the more surface sensitive low energy (55 V) Ba Auger transition, also showed results very similar to the ISS measurements. Sputter profile data taken of a surface which had been activated to 1200 K is shown in fig. 7. The strong Ba concentration which had built up at the surface during this activation is very apparent from the ISS concentration changes with depth, but it is barely seen with the standard (590 V) AES peak. In cathodes measured earlier in life, where the background Ba concentration was 60% or above, any concentration changes due to such Ba surface layers were even more obscure in the 590 V AES data. The 55 V Ba AES peak data and the ISS data, on the other hand, always showed the very distinct presence of surface layer of Ba, as is shown in fig. 8. Figs. 8a and 8b show variations hi Ba concentration profile as the cathode is heated to different activation temperatures after sputter cleaning. Fig. 8a shows ISS results on a cathode similar to that described in fig. 7 except in a later stage of life (i.e., somewhat lower bulk Ba concentration) while fig. 8b gives 55 V AES results on a fairly new cathode. The progressive build-up of the surface Ba layer is clearly visible from both the AES and ISS data as the activation temperature is raised. Since the greater AES surface sensitivity of the Ba 55 V peak compared to the 590 V peak has been established by comparison to simultaneous ISS data, it seems worthwhile to make use of data involving the ratio of these two peaks in order to study the Ba concentration gradient. Furthermore, changes in such concentration gradients can be obtained quickly and easily during an experiment without the need

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for introducing gas ions as were needed in the ISS experiments. Most importantly, concentration gradient information can be obtained from the multiple Auger energies without the destructive effects of sputter profiling. Except for the qualitative observation that an increase in the 55 V/590 V peak height ratio indicates an increase in surface Ba concentration compared to that deeper within the bulk no oth-

A. Shih, G.A. Haas / Poisoning and reactivation in oxide-type cathodes. Part 1.

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er quantitative interpretation of this ratio will be made here Fig. 9 is a plot of the 55 V/590 V AES peak height ratio for two experiments (circles and squares) after heating cathodes with active Ni to ‘~l300K following sputtering. It is seen that the magnitude of the Ba concentration gradient decreases in about 50 A to the point where there is not much change in Ba concentration from the bulk value, which is in agreement, for example, with ISS sputter profile data of fig. 7. Data on a chemically passive substrate of Ir, however, show no changes in concentration gradient and indicate the absence ofsuch a Ba surface layer. ‘1’S

5. Miscellaneous reactivation effects following ambient exposure on cathodes with active Ni substrates In previous sections, results from studies using SLEEP, SAM, ISS, work function and gas desorption showed the important role that the pick-up of C compounds (e.g., ambient CO2) and their decomposition play in the shelf-life poisoning and reactivation properties of oxide-type cathodes (sections 2 and 3). In section 4, the effects associated with the formation of a Ba surface layer (the result of reducing additives in the substrate) were also demonstrated to be equally important in contributing to the reactivation. In this section, additional effects are presented which *

A study to relate this ratio to concentration gradients is in progress.

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were also observed from the above type of studies. These effects may not necessarily be as important in influencing work function changes as the previously mentioned ones were, nor are the interpretations of these data always as clear-cut. The results are presented here, however, for the sake of completeness. In fig. 1 Oa, the Ba concentration as obtained with ISS is plotted as a function of reactivation temperature. As before, the reactivation data were taken near room temperature after heating for ~2 mm at the temperature noted by the experimental points. The squares of fig. 1 Oa are data taken after sputter cleaning, and these show the increase in surface Ba concentration at 700—800 K resulting in the formation of a Ba surface layer, as was previously discussed in section 4. For this case, the Ba surface concentration has risen from the sputtered (bulk) value of ‘~-‘20%to a surface Ba concentration of “-‘40—50% after activation to 1200—1400 K. The circles of fig. l0a are data on a similar activated surface which is being reactivated after exposure to vacuum ambient gases for several hours. The Ba surface concentration upon reactivation remains fairly constant in the 40—50% range until the cathode is heated in the vicinity of 700—800 K, at which time the surface Ba concentration rises to the anomalously high 80—90% range. Upon further heating, the Ba surface concentration slowly decreases until the 40—50% equilibrium value for the surface Ba layer is again reached at ‘~‘1350 K. In fig. lob, the 55 V/590 V ratio of the Ba AES lines is plotted for reactivation following sputtering (circles) and ambient exposure (squares and diamonds) much as was the case in fig. lOa. The increase in this ratio with increasing activation temperature after sputter cleaning the surface (circles), signifies an increase in the magnitude of the Ba concentration gradient at the surface (i.e., greater Ba concentration on the surface than just inside). This corroborates results presented in section 4, such as the ISS sputter profile data of fig. 8a which shows the build-up of this concentration gradient with reactivation temperature. Since both ISS and AES ratio data identify the activation following sputtering with the appearance of Ba on the surface (rather than in the bulk), one is led to infer that this activation is due to excess surface Ba (which originated at the interface of the oxide with the substrate) and was very likely transported to the emitting surface via diffusion over the surfaces of the individual crystallites comprising the porous cathode, instead of by diffusion through the bulk crystallite. The data given by the squares and diamonds of fig. lob, which show results after ambient exposure, indicate that the anomalously high increase in ISS Ba concentration noted by the circles of fig. 1 Oa is indeed real. Moreover, because of the corresponding increase in the surface concentration gradient, this increase is likely caused by the addition of Ba atoms again arriving predominantly by transport outside the crystallite bulk such as a surface or pore transport process. The fact that the magnitude of this gradient decreases more rapidly than the ISS surface concentration as the temperature is raised could be the result of some of the surface Ba diffusing into the crystallites at the higher temperatures, thus decreasing the gradient near the surface. This effect is certainly suggested from sputter profile data such as in fig. 8b.

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A. S/i/i, G.A. Haas/Poisoningand reactivation in oxide-type cathodes. Part I. OXIDE CATHODE ACTIVATION FOLLOWING AMBIENT EXPOSURE 1KV Ne ISS HYDROCARBONSI’I

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It is also interesting to note that no changes in concentration gradient were observed for BaO on non-reducing substrates such as Ir or substrates where the interface is already saturated with oxygen, as was the case for Ta. Examples of changes in concentrations of chemical constituents other than Ba is shown in figs. 11 and 12. Actual ISS spectra showing the reflected ion current plotted as a function of energy in fig. 11 shows the characteristic peaks associated with Ne ions having given off momentum to surface Ba and Sr atoms; also included at the low energies are a series of peaks, not due to reflected Ne ions, but commonly associated with the presence of hydrocarbons [20]. The typical rise in the Ba peak with reactivation is apparent from these data. Also of interest in this reactivation is a dramatic decrease in the Sr peak followed by an almost complete disappearance near 920 K with a reappearance at high temperatures. The hydrocarbon peak also is seen to disappear, but more complete data of fig. 12 will show an actual maximum occurring before its disappearance. The data in fig. 1 2a show variations in the Sr ISS concentration during reactivation following ambient exposure of the type described in fig. 11. The results show good reproducibility for two experiments and also show in better detail the mini-

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mum in Sr ISS concentration mentioned in fig. 11. Figs. 1 2b and 1 2c show the Cl and S AES concentration during a similar type reactivation as in fig. 1 2a. (AES was used here since the light and similar mass of Cl and S did not allow them to be distinguished with Ne ISS.) Because of the inverse nature of the Cl concentration with respect to the Sr, it was initially supposed that the Cl coming to the surface might be preferentially covering the Sr before desorbing and thus account for the minimum in the Sr concentration. Two factors, however, discourage this supposition; namely that (granted the AES data is not as surface sensitive as ISS) it is still hard to see how the actual surface Cl concentration could change enough to cause such a large change as is seen in the Sr concentration. Secondly, the anomalously high Cl peak was traced to impurities in a section of a gas inlet manifold used in the ISS experiment. When the Cl was substantially removed, the Sr effect was still observed. It is now felt that unless it is completely fortuitous, the Cl is probably another effect of whatever is responsible for the Sr to change, rather than the cause of it. The S effect at the higher temperatures (fig. 1 2c) is a reversible surface segregation from the substrate and is typical of what has been reported previously [151. The data of fig. 1 2d is a plot of the low energy (hydrocarbon) desorption peak of the ISS data mentioned in fig. 11. Since the ordinate in fig. 1 2d was obtained just from the area under this low energy peak, no absolute concentration data were obtainable. (Hopefully, forthcoming secondary ion mass spectrometry, SIMS, measurements will be able to identify these hydrocarbons as well as to assign some quantitative value to their concentration.) It is felt, however, that these hydrocarbons, which are only seen in the ISS spectra after ambient exposure, are probably related to the behavior of the Sr and Cl as well as the anomalous surface Ba reported in figs. lOa and lob. A possible model to explain the result of fig. 10 is that upon heating to 700— 800 K, hydrocarbons which may be composed in part of methane as well as mixtures of carbonates and hydroxide decomposition products, are released from the porous oxide matrix and start leaving the surface. This release is not only evident from the ISS results of fig. I 2d but also from the gas desorption data of fig. 5. By some mechanism not yet understood, the release of these hydrocarbons(?) either enhances the diffusion of Ba or may actually help transport it to the surface in form of a compound (e.g., BaCO3). The Sr, on the other hand,is then possibly covered up by either these hydrocarbons, Ba compounds, or combinations before they are finally desorbed from the surface leaving only the mixed alkaline earth oxide with a slight Ba surface layer of the type which is formed byjust heating a sputtered surface. The composition of this surface layer represents some type of dynamic equilibrium between a generation from the substrate, transport to the emitting surface via pores and crystallite surface diffusion, diffusion into the bulk crystallite and desorption from the surface.

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A. Shih, G.A. llaas / Poisoning and reactivation in oxide-type cathodes. Part!.

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6. Summary The following summarizes the major conclusions discussed in the first part of this work: (a) Oxide cathodes, like impregnated cathodes, will poison in a few hours after shut-down in a typical vacuum tube ambient (fig. 1). Successful turn-on techniques must not only be able to bring the cathode up to temperature, but also to aclueve complete reactivation from the poisoned state in the required turn-on time. (b) The build-up of surface carbon seems to be a contributing factor in increasing the work function of oxide cathodes as they are exposed to ambient tube gases during tube shut-down. Removal by electron desorption of some regions of this C wifi cause the underlying (Ba/Sr) 0 to reappear in those regions and the work function there to be decreased back to the non-poisoned value (fig. 2). (c) Reactivation is normally accomplished by heating. Beyond 750 K surface carbon starts to desorb and at ‘—1200—1300 K the work function returns to the active state value (figs. 3,4). Gas analysis measurements show desorption of CO2 (and also H20) during this activation which is consistent with carbonate and hydroxide decomposition (fig. 5). (The build-up of a hydroxide layer from H20 exposure as well as the take-up of 0 by the lattice from 02 exposure is not as visible froni simple AES measurements as is the pick-up of C for CO2 exposure described here. The effects are detectable, however, with other techniques described in part II, and it will be shown there that they are equally as important as the CO2 poisoning.) (d) Additional activation effects occurring simultaneously with the hydroxide and carbonate decomposition involve the formation of a Ba surface layer. Although this Ba surface layer has been postulated to explain activation effects for many years, even modern AES techniques as a rule are not sufficiently surface sensitive to readily distinguish this excess Ba on the surface from normal variations of the bulk background Ba concentration (fig. 7). It was detected, however, with SLEEP combined with ISS and special techniques comparing low to high energy AES peaks (figs. 6--b). (e) The Ba surface layer is formed from excess Ba generated at the interface by the action of reducing additives in active Ni substrates. The Ba, once generated, then is transported to the emitting surface via diffusion over crystallite surfaces or pores rather than by bulk crystal diffusion. (f) An increase was observed in the transport of Ba to the cathode surface whenever the cathode l1ad been heated after shut-down (fig. 10). It is speculated that this transport is related to the release of carbon (or hydrocarbon) products to the surface from the porous interior of the cathode (fig. 12). It is clear that while the work described here gives a good insight on some of the phenomenological surface effects during actual tube shut-down and reactivation using realistic oxide-type cathodes, further studies to identify and understand the basic mechanism of these phenomena will require exposures of well-defined gas dosages on equally well~characterized(e.g., single crystal) surfaces. These studies comprise the second phase of this work.

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References [1] G.A. l-Iaas, J.T. Jensen, C. [br and A. Shih, Appl. Surface Sci. 5 (1980) 73. [2] A. Shih, GA. l-laas, J.T. Jensen and C. Hor, Appl. Surface Sci. 8 (1981) 108. [3] G.A. Haas and RE. Thomas, Surface Sd. 4 (1966) 64; J. Vacuum Sci. Technol. 13 (1976) 479. [41 C. Herrmann and S. Wagener, The Oxide Coated Cathode, Vol. 2 (Chapman and Hall, London, 1951). [5] P. Zalm,Advancesin Electronics and Electron Physics (Academic Press, New York, 1968). [6] TN. Chin, R.W. Cohen and M.D. Couttes, R.C.A. Rev. 35 (1974) 520. [7] A.H.W. Beck, A.B. Cutting, A.D. Brisbane and C. King, Nature 174 (1954) 1010. [8] C P. Hadley and W.G. Rudy, J. Electrochem. Soc. 105 (1958) 395. [9] R.W. Lane, Brit. J. Appl. Phys. 9 (1958) 149. [10] D.W. Maurer and CM. Pleass, Bell Syst. Tech. J. 2375 (1967). [11] V.E. Rutter, D.C. Schimmel, R.A. Handy and B.H. Smith, Appl. Surface Sci. 2 (1979) 118. [12] C.G. Pantano and TN. Wittberg, Appl. Surface Sci. 4(1980) 385. [13] GA. Haas, A. Shih and R.E. Thomas, Appl. Surface Sci. 1(1977)59. [14] A. Shih, C. Hor and GA. Haas, Appl. Surface Sci. 2 (1979) 112. [15] G.A. Haas,A. Shih and R.E. Thomas, AppI. Surface Sci. 2(1979)293. [161 J.A.Th. Verhoeven and H. van Doveren, Appl. Surface Sci. 8 (1981) 95. [17] J.J. Lander, J. Am. Chem. Soc. 73(1951)5794. [18] G.A. Haas and J.T. Jensen, Jr., Rev. Sci. lnstr. 30 (1959) 562. [19] G.F. Huttig and A. Arbes, Z. Anorg. Allg. Chem. 196 (1931) 403. [20] 3-M Analytical Systems Application Notes, No. 524 (1977).